1. Field of the Invention
The present invention relates to a read head including a spin valve sensor with a specular reflecting cap layer structure and, more particularly, to such a spin valve sensor wherein the cap layer structure minimizes sense current shunting while maximizing specular reflection of conduction electrons.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer structure and a ferromagnetic free layer structure. An antiferromagnetic pinning layer interfaces the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer structure is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is preferably parallel to the ABS, is the position of the magnetic moment of the free layer structure with the sense current conducted through the sensor in the absence of signal fields.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layer structures are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with the pinned and free layer structures. When the magnetic moments of the pinned and free layer structures are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layer structures. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in the resistance of the sensor as the magnetic moment of the free layer structure rotates from a position parallel with respect to the magnetic moment of the pinned layer structure to an antiparallel position with respect thereto and R is the resistance of the sensor when the magnetic moments are parallel.
In addition to the spin valve sensor the read head includes nonconductive nonmagnetic first and second read gap layers and ferromagnetic first and second shield layers. The spin valve sensor is located between the first and second read gap layers and the first and second read gap layers are located between the first and second shield layers. In the construction of the read head the first shield layer is formed first followed by formation of the first read gap layer, the spin valve sensor, the second read gap layer and the second shield layer. Spin valve sensors are classified as a top or a bottom spin valve sensor depending upon whether the pinning layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic layers that are separated by a coupling layer with magnetic moments of the ferromagnetic layers being antiparallel. Spin valve sensors are still further classified as single or dual wherein a single spin valve sensor employs only one pinned layer and a dual spin valve sensor employs two pinned layers with the free layer structure located therebetween.
The spin valve sensor has a spin scattering region which is located between the aforementioned interfaces of the spacer layer with each of the pinned and free layer structures. As indicated hereinabove, increases and decreases in scattering of the conduction electrons through this spin-dependent scattering region results in increases and decreases of resistance of the spin valve sensor to the sense current. Unfortunately, a portion of the conduction electrons escape from the spin scattering region which reduces the aforementioned magnetoresistive coefficient dr/R. A scheme for preventing escape of these conduction electrons from the spin-dependent region is to provide a specular reflector layer in a cap structure at the top of the spin valve sensor. The specular reflector layer reflects the conduction electrons back into the spin-dependent region so as to improve the magnetoresistive coefficient dr/R of the sensor.
The present invention provides a bilayer cap structure for a spin valve sensor which minimizes shunting of the sense current through the spin valve sensor while maximizing reflection of conduction electrons back into the spin-dependent region of the sensor. The cap structure is a combination of a first layer which is composed of a nonmagnetic metal and a second layer which is composed of an iron oxide with the nonmagnetic metallic first layer being located between the free layer structure and the iron oxide second layer. Each of the first and second layers of the cap structure are specular reflectors, however, the nonmagnetic metallic first layer shunts the sense current. Accordingly, the thickness of the nonmagnetic metallic first layer is thinner than the iron oxide second layer with the preferred thicknesses being 5 xc3x85 for the first layer and 10 xc3x85 for the second layer. The nonmagnetic metallic first layer serves a dual function, namely: (1) reflecting conduction electrons back into the spin-dependent region and (2) insulating the iron oxide second layer from the free layer structure. I have found that when the iron oxide second layer interfaces the free layer structure it seriously degrades the free layer structure by raising its uniaxial anisotropy HK. When the HK of the free layer is elevated it becomes more stiff and less responsive to signal fields from the rotating magnetic disk.
An object of the present invention is to provide a cap structure for a spin valve sensor which minimizes shunting of the sense current while maximizing the reflection of conduction electrons.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.